At present, various different types of cellular mobile telecommunications networks using different technologies co-exist with each other, and further developments of cellular network technology are being planned. Well-known technologies employed in mobile telecommunications networks include GSM (“Global System for Mobile Communications”), UMTS/GPRS (“Universal Mobile Telecommunications System”/“General Packet Radio Service”), CDMA2000®, WiMax (“World Interoperability for Microwave Access”), and so on, and the last few years have seen the introduction of LTE systems (“Long Term Evolution” systems) according to release 8 or release 9 specifications developed by the 3GPP (Third-Generation Partnership Project).
Irrespective of the network technologies being used, an important issue in a mobile cellular telecommunications network is how the mobility of the user equipment (UE) is managed. Although there are differences of detail (and in terminology) between the various technologies, a common feature of mobility management is the need for:                a) a procedure whereby the UE reports its position to the network, and        b) a procedure whereby the network manages to contact the UE when a call is directed to that UE.In this document, procedures of type a) above shall be referred to as “location update” or “location registration” procedures, and procedures of type b) above shall be referred to as “paging” procedures.        
In existing cellular mobile communications networks, location updates are used to enable the network to register information designating a particular region where the UE can be reached. In general, the registered information designates a group of one or more cells covering the UE's location at the time when the location update was performed. In this document, the expression “location area” (LA) shall be used to designate such a region/group of cells, and the expression “location area identifier” (LAI) shall be used to designate a code or identifier that identifies a location area. The cells of a given location area have the same LAI. It will be understood that the invention can be used advantageously without limit to the network technology, such that the expression “location area” and “LAI” cover any comparable region and its identifier irrespective of the nomenclature (“tracking area”, etc.) used in the technology concerned.
When the network receives a call for a particular UE, a check is made of the UE's registered location area, and a paging message is broadcast in cells which have the LAI applicable to the registered location area.
It will be understood that the details of the location update procedure and paging procedure will vary between networks using different technologies. FIG. 1 illustrates steps in a typical location update procedure and FIG. 2 illustrates steps in a typical paging procedure.
In step S101 of FIG. 1 it is assumed that the UE enters an idle state at the end of a communications session. In this idle state the UE is camped on the cell which was handling communication with the UE at the time when the UE's last session ended. In the idle state, at certain times the UE monitors the radio transmissions being made by transceivers in its vicinity. This monitoring process detects different cells which could handle communications for this UE and monitors properties of the various cells including, but not limited to, signal strength. In step S102 illustrated in FIG. 1 the UE determines, based on its monitoring, whether or not the UE has entered a location area having a new LAI compared to the cell on which the UE has been camped. If the UE determines that there has not been a change of LAI then the processing returns to step S101. Alternatively, if the UE determines in step S102 that the UE has entered a location area having a new LAI then the process moves to step S103.
In step S103 the UE makes a determination of which would be the best cell to camp on from now on. The UE's assessment of which cell is “best” may be performed by any convenient metric. For example, the UE may assign a rank to each potential cell with which it could communicate, basing the rank on one or more criteria such as signal strength, interference in the cell, and so on. When the UE has selected the “best” cell the UE camps on that cell and may perform various steps including tuning to the control channel of the new cell.
In step S104 of FIG. 1, the UE communicates with the network infrastructure to register the LAI of the cell on which it is newly camped. Following procedures specified in standards applicable to various existing mobile networks, the UE performs the location update by signalling to the new cell so that a radio channel can be assigned and the UE can send a request, through the new cell, for the updated LAI to be registered by the network. The updated LAI may be registered by different components in the network. In general the LAI will be registered by a controller that manages mobility of UEs in a section of the mobile network that includes the new cell. The present description shall refer to controllers which manage mobility of UEs using the expression Mobility Management Entities (MMEs) using the same expression as is used in the LTE standard. The LAI and/or the MME where the UE is registered will also be registered in a network component which constitutes a home server or home register in respect of the subscriber who owns the relevant UE.
It will be understood that if a UE moves rapidly through several different cells belonging to different location areas then, according to the specifications of existing cellular mobile communications networks, the UE will make frequent location updates (see below).
When a call is made to a UE in the mobile telecommunications network, the registered LAI information is used so as to determine where to broadcast a paging message that may be detected by the relevant UE. The steps in a typical paging procedure are illustrated in FIG. 2.
In step S201 illustrated in FIG. 2 the network infrastructure has determined that a call is being made to a specific UE and the home server or home register for that UE is consulted so as to determine the MME where the UE in question is currently registered. In step S202 of FIG. 2, the MME where the UE is currently registered transmits a paging request to transceivers in cells of the location area whose LAI is currently registered for this UE. In step S203 of FIG. 2, the transceivers covering the cells of the location area identified by the relevant LAI broadcast paging messages. In step S204 of FIG. 2, the UE detects a paging message broadcast by a cell C of the location area and determines that the paging message is directed at itself. As a result, in step S205 the UE sends a response to cell C. Various messages are then exchanged between the UE and cell C and between cell C and the network infrastructure so as to set up the subsequent call between the UE and the calling party through cell C (step S206 in FIG. 2).
In recent years many mobile cellular communications networks have developed a hierarchical cell structure including cells in different cell-size classes, that is, including traditional cells of relatively large size, and small cells of relatively much smaller size. Although the cells of the larger cell-size class, macrocells, can have different sizes they generally cover regions whose dimensions are of the order of one or several kilometers. The cells of the smaller cell-size class are often called “femtocells” and generally cover much smaller regions, for example, regions of the order of 10 meters in diameter. Typically, femtocells are installed to improve network coverage, for example: in locations where there is a high density of users, or within a building, or at rural locations where signal coverage would be poor otherwise. Some businesses or organisations install a set of femtocells in a building or campus in order to provide good network coverage throughout the building or campus.
In this document, depending on the context, the expression “cell” is generally used to designate an access node of the radio access network portion of a mobile network, or to designate the region of radio coverage of an access node.
According to the release 8 and release 9 LTE specifications, macrocells include a component designated eNB or eNodeB (which stands for “E-UTRAN Node B” or “Evolved Node B”) which is connected to the core network and handles radio communications with the UEs in its locality. Networks according to other technologies have comparable elements: NodeB in UMTS, base station transceiver/radio network controller in GSM, and so on. The corresponding element in an LTE femtocell is designated an HeNB (which stands for “Home eNodeB”).
Often a femtocell will connect to the core network using a residential DSL (Digital Subscriber Line), cable broadband connection, optical fibre, wireless last-mile technology, or other connection using Internet Protocol connection and so on, in association with a gateway to the core network. Femtocells operate at frequencies that are licensed to specific telecommunications companies and so, in the same way as access network components, they handle voice and/or data calls for subscribers with the relevant company. Many femtocells are configured so that only specified UEs are allowed to communicate via the femtocell in question. For example, when a femtocell is installed in a home setting, the femtocell may be configured so only mobile phones belonging to family members may access the femtocell. The present description uses the expression “closed subscriber group” (CSG) to designate a list of specified UEs who are the only ones allowed to make use of a given femtocell. A given CSG may be authorized to use a group of femtocells and the femtocells of the group share a common identifier (CSG ID).
Femtocells are small and are often located in clusters, for example in urban environments. Accordingly, when a UE moves about in such an environment, the UE may move rapidly through a relatively large number of femtocells and macrocells and these may be assigned different LAIs. In such circumstances, based on the methods used in conventional mobile cellular telecommunications networks, the UE will make frequent location updates. This is undesirable because the UE uses power to send location updates and so frequent updates lead to a need to re-charge the UE's battery more often. Also, location updates involve the transmission of messages between the UE and the network infrastructure, as well as messages between components in the network infrastructure, so that frequent location updates use up bandwidth and reduce the network's capacity to handle calls.
Various techniques have already been proposed for reducing the number of location updates that will be generated in mobile cellular telecommunications networks which have a hierarchical structure (e.g. which use macrocells and femtocells). For example, it has been recognized that there are circumstances where the same LAI can be assigned to different femtocells in a locality, for example in a case where plural femtocells are installed at different locations within a building belonging to one business all of these femtocells may use the same LAI. Accordingly, there will be no change of LAI when a UE moves from one of these femtocells to another and, thus, no need to make a location update at that time.
However, femtocells which belong to different organisations or households generally are assigned different LAIs from one another, and macrocells are assigned different LAIs from femtocells. Accordingly, as a UE moves between two such femtocells, or between a femtocell and a macrocell, frequent location updates will still be needed. This problem will now be discussed with reference to FIG. 3.
FIG. 3 is a diagram illustrating, in simplified form, an example of a region R of a mobile cellular telecommunications network in which there are two macrocells designated MACRO_1 and MACRO_2, as well as four groups of femtocells, designated CSG_0, CSG_1, CSG_2 and CSG_3. The four groups of femtocells correspond to four respective closed subscriber groups. As illustrated in FIG. 3, there are three femtocells for each of the closed subscriber groups. The femtocells of groups CSG_0 and CSG_1 are all located within the coverage region of macrocell MACRO_1. The femtocells of groups CSG_2 and CSG_3 are all located within the coverage region of macrocell MACRO_2. A network component, designated MME in FIG. 3, manages the mobility of UEs in the region R. A first UE, identified by a unique identifier (designated IMSI_1 in FIG. 3), is connected to the mobile cellular telecommunications network but is not located in region R, whereas a second UE, identified by its own unique identifier (designated IMSI_2 in FIG. 3) is located in region R and moves around within region R. These UEs shall be referred to from now on using their identifiers.
In the example illustrated in FIG. 3, a single location area is assigned to the two macrocells MACRO_1 and MACRO_2 and so these macrocells both have the same LAI, designated LA_1 in FIG. 3.
In the example illustrated in FIG. 3, a single location area is assigned to the femtocells of groups CSG_0 and CSG_1 and so the femtocells of groups CSG_0 and CSG_1 all have the same LAI, designated LA_2 in FIG. 3. It will be noticed that the femtocells of groups CSG_0 and CSG_1 are not assigned to the same location area as the macrocell MACRO_1 even though femtocells of groups CSG_0 and CSG_1 are located within the coverage region of macrocell MACRO_1.
In the example illustrated in FIG. 3, a single location area is assigned to the femtocells of groups CSG_2 and CSG_3 and so the femtocells of groups CSG_2 and CSG_3 all have the same LAI, designated LA_3 in FIG. 3. In this case also, it will be noticed that the femtocells of groups CSG_2 and CSG_3 are not assigned to the same location area as the macrocell MACRO_2 even though the femtocells of groups CSG_2 and CSG_3 are located within the coverage region of macrocell MACRO_2.
Consider a case where IMSI_2 is initially located at the position marked {circle around (1)} in FIG. 3 and moves along a path indicated by the dashed arrow A in FIG. 3 to the position marked {circle around (2)} in FIG. 3. At the time when IMSI_2 starts moving at position {circle around (1)}, the last communication session IMSI_2 engaged in has terminated with IMSI_2 attached to a femtocell of group CSG_3. The mobility management entity MME of the network has LA_3 registered to identify the location area of IMSI_2. IMSI_2 is camped on one of the femtocells of group CSG_3 but can detect transmissions from the other femtocells of CSG_3 and from the macrocell MACRO_2.
When IMSI_2 moves out of range of the femtocells of CSG_3 travelling along path A, it enters an area where IMSI_2 is only reachable by MACRO_2. IMSI_2 chooses to camp on cell MACRO_2 and detects that the LAI for MACRO_2 is different from the LAI of the cell on which IMSI_2 was previously camped. Accordingly, IMSI_2 communicates with MACRO_2 to request performance of a location update that will register the LAI of MACRO_2 (i.e. LA_1) to the applicable mobility management entity MME. As IMSI_2 continues to move along path A it enters an area covered by the femtocells of CSG_2 and determines that it should camp on a femtocell of CSG_2. IMSI_2 detects that the LAI of the chosen CSG_2 femtocell is different from the LAI of the cell on which IMSI_2 was previously camped, and so communicates with the chosen CSG_2 femtocell to request performance of a second location update, this time to register LA_3 to the MME.
When IMSI_2 moves out of range of the femtocells of CSG_2 travelling along path A, it enters an area where, once again, IMSI_2 is only reachable by MACRO_2. IMSI_2 chooses to camp on cell MACRO_2, detects that the LAI for MACRO_2 is different from the LAI of the cell on which IMSI_2 was previously camped and communicates with MACRO_2 to request performance of a third location update, on this occasion to register LA_1 to the mobility management entity MME. As IMSI_2 continues to move along path A it enters an area where IMSI_2 is only reachable by MACRO_1. IMSI_2 chooses to camp on cell MACRO_1, but detects that the LAI for MACRO_1 is the same as the LAI for the previous cell on which IMSI_2 was camped and so no location update is required. Accordingly IMSI_2 does not send a location update request at this time.
As IMSI_2 continues to move along path A it enters an area covered by the femtocells of CSG_1 and determines that it should camp on a femtocell of CSG_1. IMSI_2 detects that the LAI of the chosen CSG_1 femtocell is different from the LAI of the cell on which IMSI_2 was previously camped, and so communicates with the chosen CSG_1 femtocell to request performance of a fourth location update, this time to register LA_3 to the MME. When IMSI_2 moves out of range of the femtocells of CSG_1 travelling along path A, it enters an area where, once again, IMSI_2 is only reachable by MACRO_1. IMSI_2 chooses to camp on cell MACRO_1 and detects that the LAI for MACRO_1 is different from the LAI of the cell on which IMSI_2 was previously camped. Accordingly, IMSI_2 contacts MACRO_1 to request performance of a fifth location update, to register the LAI of MACRO_1 (i.e. LA_1) to the mobility management entity MME. As IMSI_2 continues to move along path A it enters an area covered by the femtocells of CSG_0 and determines that it should camp on a femtocell of CSG_0. IMSI_2 detects that the LAI of the chosen CSG_0 femtocell is different from the LAI of the cell on which IMSI_2 was previously camped, and so contacts the chosen CSG_0 femtocell to request performance of a sixth location update, this time to register LA_2 to the MME.
It will be understood from the above explanation that the location update techniques that are applied in conventional mobile cellular networks may require a large number of location updates to be performed as a UE moves around in a region covered by macrocells and femtocells. In the example discussed above with reference to FIG. 3, IMSI_2 performs 6 location updates as it moves along path A from location {circle around (1)} to location {circle around (2)}.
Considering paging procedures in the region R illustrated in FIG. 3: if IMSI_1 places a call to IMSI_2 when IMSI_2 is at position {circle around (1)} at the end of path A, the MME has LA_2 registered as the identifier of the location area for IMSI_2 and so directs a paging request to the radio access network components covering the cells having the location area identifier LA_2. In this case, the femtocells of CSG_0 and CSG_1 are the cells having the location area identified LA_2 and so they respond to the paging request from the MME by broadcasting paging messages which include an identifier of IMSI_2. Let us say that IMSI_2 is camped on a femtocell FC of CSG_0 and detects a paging message identifying IMSI_2, broadcast by femtocell FC. In this case IMSI_2 sends its response to femtocell FC and a session connecting IMSI_1 to IMSI_2 is then established through femtocell FC.
In the previous example, when IMSI_2 is at position {circle around (2)} in FIG. 3 the MME sends requests for paging messages to be broadcast to IMSI_2 from six femtocells, that is the femtocells of CSG_0 and CSG_1. In the case of closed subscriber group femtocells, it is possible to obtain a reduction in the number of femtocells broadcasting paging messages by taking into account which CSGs the called UE belongs to. For example, if IMSI_2 in FIG. 3 is only a member of CSG_1 and CSG_3 then paging overhead may be reduced by making sure that paging messages are broadcast only in femtocells which, as well as having the appropriate LAI, also belong to a CSG that IMSI_2 is authorized to access. Thus, in the previous example, when IMSI_2 is at position {circle around (2)} in FIG. 3 the MME could request paging messages to be broadcast to IMSI_2 from only the three femtocells of CSG_1 because, although the femtocells of CSG_0 are in the appropriate location area, IMSI_2 is not authorized to access CSG_0.
In conventional mobile cellular communications networks, if a single location area includes a large number of macrocells or femtocells this can reduce the number of location updates that are performed but, conversely, a large number of cells may need to broadcast paging messages when a call is directed to a UE in this location area. However, if a single location area includes only a small number of macrocells or femtocells then, although this can reduce the paging load, there is an increased requirement for frequent location updates.
The present invention has been conceived in the light of the above problems.